perf(workspace): harden memory lifecycle, staging buffer disposal, and grid row cache cleanup

[ZhuchkaTriplesix]

Closes #670

Summary of Changes

• DataGridStagingBuffer: Implemented explicit dispose() override that clears modified cells, inserted rows, and deleted row indices, immediately freeing heap allocations when staging is reset or discarded.
• Workspaces (PostgresSqlWorkspace, MysqlSqlWorkspace, SqliteSqlWorkspace, ExtensionTableView):
  • Added explicit _stagingBuffer?.dispose() before allocating a new staging buffer on subsequent query executions, mutation saves, and table pagination.
  • Added clean disposal of _stagingBuffer in workspace dispose() lifecycle methods.
• ResultsTab: Added dispose() override in _ResultsTabState to unbind memoized filtered rows (_cachedFilteredRows, _memoEffectiveRows) upon unmounting.
• VirtualResultGrid: Added cache resets (_sortedRows = const [], _columnWidths = const [], _columnOffsets = const [0]) in VirtualResultGridState.dispose() to immediately release large matrix references to the GC.
• Unit Tests: Added test in data_grid_staging_buffer_test.dart validating that dispose() clears internal collections and resets change tracking.